ARACEAE 
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disc. Usual formula K4, C 4, A 4, 0(4), 4-locular, with 1 or 2 
pendulous anatropous ovules in each. Drupe. Endosperm. Chief 
genus: Ilex. [A. = Ilicineae B. and H. (coh. Olacales); placed in 
Frangulinae by Warming.] 
Aquilegia (Tourn.) Linn. Ranunculaceae (2). 50 sp. N. temp. 
(A. vulgaris L., the columbine, in Brit.). The petals are prolonged 
into long spurs (cf. Delphinium), secreting honey that is only ac- 
cessible to long-tongued insects. Sta. often 50 or more, in whorls 
of 5. Fir. protandrous, fertilised by humble-bees. 
Arabis Linn. Cruciferae (iv. 15). 100 sp. N. temp., S. Am. 5 Brit, 
(rock-cress). 
Araceae ( = Aroideae Benth. -Hooker). Monocotyledons (Spathiflorae). 
105 gen. with abt. 1000 sp. trop. and temp, but chiefly trop. 
(92 °/ 0 ). Many types of vegetative habit occur in the order — herbs 
large and small, with aerial stems, tubers or rhizomes, climbing 
shrubs, climbing epiphytes, marsh plants, one water plant (Pistia) &c. 
In a few Pothoideae (the oldest group) the stem is monopodial, but 
in most A. it is sympodial. Each joint of the sympodium begins as 
a rule with one or more scale leaves before bearing foliage leaves. 
Accessory (collateral) buds are often found in the leaf axils. Some- 
times, as in Anthurium, Philodendron &c., the axillary shoot is 
‘ adnate ’ to the main axis for some distance (cf. Solanaceae, Zostera 
&c.). The buds usually appear in the leaf axils, but often get 
pushed to one side, and sometimes {e.g. Pothos) break through the 
leaf-bases as in Equisetum. 
The leaves show many types. Pinnately and palmately divided 
leaves are frequent, but their development is not like that of such 
leaves in Dicotyledons. Holes are present in the leaves of Monstera. 
See Alai. PJl. for details, and genera Monstera, Rhaphidophora, 
Philodendron, Helicodiceros, Dracontium, Zamioculcas &c. 
The roots are adventitious and mostly formed above ground in 
the larger forms. Two types of aerial root occur — climbing roots 
and absorbent roots. The former, like those of ivy, are insensitive to 
gravity but show great negative heliotropism ; they consequently 
cling closely to the support and force their way into all the crevices 
of its structure. The latter are insensitive to light, but respond 
markedly to gravity ; they therefore grow down to the soil and enter 
it, branching out below ground and taking up nourishment. 
The larger tropical A. show interesting stages in the development 
of epiphytism (see p. 173). The climbing forms grow to considerable 
size and form longer and longer aerial roots as they grow upwards. 
The original roots at the base of the stem thus become of less and 
less importance and it not uncommonly happens that they die away 
together with the lower end of the stem, so that the plant thus 
becomes an epiphyte. Of course, as it still obtains its water <fcc. 
from the soil, it is not an epiphyte in the sense that e.g. many 
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